Chronic myelogenous leukemia (CML) is characterized by a t(9;22) chromosomal translocation, which results in a fusion gene whose protein product shows enhanced tyrosine kinase activity. This fusion protein is a target in a new promising treatment using a Bcr-Abl kinase inhibitor. CML is typically detected using either slide-based fluorescence in situ hybridization (FISH) to detect the translocation, or reverse transcriptase-polymerase chain reaction (RT-PCR) to detect the fusion mRNA transcript. FISH is labor-intensive and cannot easily detect rare cancer cells. RT-PCR is susceptible to contamination and false positive results. This Phase I research proposes to develop a novel assay format for detecting CML positive cells. Use of flow cytometry will permit analysis of at least a million cells per hour, and identification of rare cancer cells. PROPOSED COMMERCIAL APPLICATION: This assay will facilitate post-treatment patient monitoring and early detection of minimal residual disease (MRD).